Citigroup is furiously trying to minimize disruption of its business following the collapse of 7 World Trade Center, where it was the biggest tenant.

The firm said some of its operations were obliterated when the building imploded. The firm occupied 39 floors in the building but noted that the “vast majority” of Citigroup’s operations are located in TriBeCa and Uptown.

The 47-story building, also known as the Salomon Brothers Building, crumbled after being weakened by an attack on the Twin Towers, just a few hundred yards away.

“Citigroup is invoking a continuity of business plans for offices located in lower Manhattan,” Citigroup, the parent company of Salomon Smith Barney, Salomon Brothers’ predecessor firm, said in a statement.

7 World Trade Center had been successfully evacuated earlier in the day. Alexandra Wollowick, a manager for Aramark Corp., a catering service for Salomon, was in the building at the time the planes crashed into the World Trade Center.

“It was about 8:45 a.m. I was in the kitchen on the second floor of the building when the building shook,” she recalls. “When I looked out the window, I saw sparks flying outside.”

“After the second plane hit, they started to evacuate us,” she added.

Among the other top tenants in building were the Securities & Exchange Commission, the Standard Chartered Bank, ITT Hartford Insurance Group, the Internal Revenue Service and the U.S. Secret Service. The Mayor’s office of emergency management had also developed a 23rd floor bunker as a safe haven.